1. Field of the Invention
The present invention relates to a line driver, more particularly to a line driver capable of automatic adjustment of output impedance.
2. Description of the Related Art
A transmission line is used to transmit high-speed signals, and a line driver is used to drive the input to the transmission line with a higher than normal signal level. In order to reduce signal reflection, an output impedance of the line driver must be matched with a characteristic impedance of the transmission line.
The line driver is adapted to be a part of an integrated circuit. Therefore, the output impedance of the line driver may deviate from a designed value due to manufacturing drifts of the integrated circuit, such that the output impedance of the line driver does not match with the characteristic impedance of the transmission line. In order to alleviate the effect of manufacturing drifts of the integrated circuit, many means have been proposed heretofore for adjusting the output impedance of the line driver.
U.S. Pat. No. 7,119,611 discloses an on-chip calibrated source termination for voltage mode driver. An amplifier is disclosed having an internal amplifier with a first output and a second output, the first output interfaced to a non-inverting input through an interface. The second output is coupled to the first output through a series resistance element. The output impedance of the amplifier is determined by the ratio of the current drive of the first and second outputs. The voltage on the second output is a function of the interface and the current input to the internal amplifier.